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PET-based confirmation of orientation sensitivity of TMS-induced cortical activation in humans
Todd D Krieg1, Felipe S Salinas, Shalini Narayana
1Department of Biomedical Engineering, Illinois Institute of Technology, Wishnick Hall 314, 3255 S. Dearborn St., Chicago, IL 60616, USA.
Background:
Currently, it is difficult to predict precise regions of cortical activation in response to transcranial magnetic stimulation (TMS). Most analytical approaches focus on applied magnetic field strength in the target region as the primary factor, placing activation on the gyral crowns. However, imaging studies support M1 targets being typically located in the sulcal banks.
Objective/Hypothesis:
To more thoroughly investigate this inconsistency, we sought to determine whether neocortical surface orientation was a critical determinant of regional activation.
Methods:
MR images were used to construct cortical and scalp surfaces for 18 subjects. The angle (θ) between the cortical surface normal and its nearest scalp normal for ~50,000 cortical points per subject was used to quantify cortical location (i.e., gyral vs. sulcal). TMS-induced activations of primary motor cortex (M1) were compared to brain activations recorded during a finger-tapping task using concurrent positron emission tomographic (PET) imaging.
Results:
Brain activations were primarily sulcal for both the TMS and task activations (P < 0.001 for both) compared to the overall cortical surface orientation. Also, the location of maximal blood flow in response to either TMS or finger-tapping correlated well using the cortical surface orientation angle or distance to scalp (P < 0.001 for both) as criteria for comparison between different neocortical activation modalities.
Conclusion:
This study provides further evidence that a major factor in cortical activation using TMS is the orientation of the cortical surface with respect to the induced electric field. The results show that, despite the gyral crown of the cortex being subjected to a larger magnetic field magnitude, the sulcal bank of M1 had larger cerebral blood flow (CBF) responses during TMS.
Insights
Cortical surface orientation, not magnetic field strength, dictates brain activation during transcranial magnetic stimulation (TMS). Sulcal bank activation in the primary motor cortex (M1) shows greater cerebral blood flow (CBF) responses with TMS.
Area of Science:
- Neuroscience
- Medical Imaging
- Electrophysiology
Background:
- Predicting precise cortical activation from transcranial magnetic stimulation (TMS) remains challenging.
- Current models often prioritize applied magnetic field strength, predicting gyral crown activation.
- However, neuroimaging data suggests primary motor cortex (M1) targets are frequently located in sulcal banks.
Purpose of the Study:
- To investigate the role of neocortical surface orientation in determining regional brain activation during TMS.
- To resolve the discrepancy between predicted gyral activation and observed sulcal activation patterns.
Main Methods:
- Constructed cortical and scalp surfaces from MR images of 18 subjects.
- Quantified cortical location (gyral vs. sulcal) using the angle between cortical and scalp surface normals.
- Compared TMS-induced M1 activations with task-based activations (finger-tapping) using concurrent PET imaging.
Main Results:
- Both TMS and task-based activations were predominantly sulcal (P < 0.001).
- Maximal blood flow locations for both TMS and finger-tapping correlated strongly with cortical surface orientation and distance to scalp (P < 0.001).
Conclusions:
- Neocortical surface orientation is a critical determinant of TMS-induced cortical activation.
- Despite higher magnetic field strength on gyral crowns, sulcal banks of M1 exhibited greater cerebral blood flow (CBF) responses during TMS.
